Abstract: The dynamic similarity between the waves in Shelf Ocean and Equatorial Area is discussed. Like the waves in Equatorial Area, the waves in Shelf Ocean consist of inertia-gravity waves, Kelvin waves, Rossby waves and conditional Rossby-gravity mixed waves. Several approximations are commonly used when resolving problems in Shelf Ocean, which inchide long-wave approximation, non-divergence approximation and low-frequency approximation. The advantage, disadvantage and the degree of these approximations are also studied in detail. Its effect in propagation of El Niño signal in ocean is also discussed.